Seals off the Pacific coast of North America have learnt the calls of whales that will not harm them, so they don't waste time fleeing unnecessarily, Scottish and Canadian researchers have found.

In a study investigating the response of harbour seals to the underwater calls of different populations of killer whales, Dr Volker Deecke of the Vancouver Aquarium Marine Science Centre in Canada and colleagues at Scotland's University of St Andrews, found that the seals don't react to the calls of non-threatening fish-eating killer whales.

But they respond strongly to calls from mammal-eating killer whales, as well as the calls of fish-eating killer whales from elsewhere, according to a report in this week's issue of the journal Nature.

"It certainly makes the point that if the seals hear a predator that they know is local, they are acclimatised. If they hear anything else, they will go," said Dr Nick Gales, an expert on marine mammals with the Australian Antarctic Division in Hobart. "That in itself is an interesting finding."

Two types of killer whales live in the Pacific off the northeast American coast. Resident killer whales live in large social groups and feed on fish, posing no threat to seals, which are mammals. Transient killer whales, however, live in smaller social groups and prey only on marine mammals, including seals.

The researchers conducted two playback experiments with harbour seals, Phoca vituliana. In the first, they played recordings of the transient whales at a harbour where the seals usually surface onto land. The group then counted the number of seals on land after playing the recordings.

Seals reacted to the transient calls by diving deep below the waters; seals near the surface are highly visible and present a good echo-location target for predators. Seals, however, did not respond to the other sounds the researchers used as controls in the experiment.

The authors speculated that the reaction is either because the seals perceive the whales as dangerous or because the sound is unfamiliar.

In the second experiment, they compared the seals' reaction to sounds of resident whales and transient whales. As expected, the seals responded to the transient whale sounds. But they also responded to sounds of resident killer whales that had been recorded elsewhere, and would have been unfamiliar to the seals.

"The fact that the seals responded strongly to the calls of fish-eating killer whales with which they had no experience shows that the difference in the response is due to selective habituation to the calls of harmless resident killer whales rather than to associative learning," the authors said.

Resident killer whales are highly vocal, with a complex system of vocal dialects. "Different social groups have repertoires of seven to 17 structurally-distinct stereotyped calls," they said.

But transient whales are less vocal than resident groups, said Dr Gales: "And that is one of the holes in the argument. The animals may vocalise when they are not hunting - but when they are in a serious search pattern, they might go entirely silent, so what is reported in this paper is not entirely relevant."

No-one has done the type of recording and observational study that would be required to find out the vocalisation patterns of killer whales during hunting, he added.

In Australia, there are no resident pods of whales as populations tend to be transient. Seals are considered a problem around fish farms, and in the past fish farmers have tried to scare them off by using recordings of predators, said Gales. But once the seals realise there is no real threat, they return.

"In that instance, effectively you are training the seals the same way [as what is occurring in the paper], to hear a vocalisation that they can ignore without any threat to them," he said.